Control arrangements for induction motor drives may comprise those, which implement constant V/f control in open loop mode. Such control arrangements may potentially operate the motor drives at enhanced efficiency, by incorporating MTPA (maximum torque per ampere) control in scalar mode. The MTPA control is a model based approach using current angle control. Typically, the stator currents of the motor are resolved into equivalent d and q axis currents and then the d axis current reference is controlled which maximizes motor efficiency.
FIG. 1 is a schematic diagram of a control system 11 for an electric three-phase variable speed motor 12 comprising such a scalar based control arrangement 14 for controlling an inverter 13, which provides power to the motor 12. The control system comprises also current sensors 15 configured to detect currents of the motor. The control arrangement comprises a d and q axis currents determining module 16 configured to determine the d and q axis currents isd, isq based on the detected currents and a reference phase θ*, a low pass filter block 17a, an MTPA control block 17b, and a switching signal generation module 18.
The current sensors 15 may be configured to detect two ia, ib of the three currents ia, ib ic of the three-phase motor 12. The d and q axis currents determining module 16 may in such instance comprise a first current determining block 16a configured to determine the three currents ia, ib ic based on the detected currents ia, ib and a second current determining block 16b configured to determine the d and q axis currents isd, isq based on the three currents ia, ib ic and the reference phase θ*.
The q axis current isq is input to the low pass filter block 17a from the d and q axis currents determining module 16 and is low pass filtered for control stability in the low pass filter block 17a to form the reference q axis current i*sq, which is input to the switching signal generation module 18.
The d and q axis currents isd, isq are input to the MTPA control block 18, which is configured to generate a reference d axis current i*sd based on the determined d and q axis currents isd, isq and a ratio value such that the ratio of the d and q axis currents is equal to the ratio value, wherein the ratio value is set to unity.
The switching signal generation module 18 is configured to generate PWM modulated switching signals to control the inverter 13 based on the reference d and q axis currents i*sd, i*sq. The switching signal generation module 18 comprises a first block 18a, to which the reference d and q axis currents i*sd, i*sq and a reference speed ω* are input. The first block 18a is configured to determine the reference d and q axis voltages u*sd, u*sq from reference d and q axis currents i*sd, i*sq. These voltages are input to a second block 18b, which is configured to repeatedly generate voltage vectors u*sa, u*sb, u*sc based on the d and q axis voltages u*sd, u*sq and a reference phase θ*. The voltage vectors are PWM modulated in a third block 18c to form PWM modulated switching signals Sa, Sb, Sc, and the PWM modulated switching signals Sa, Sb, Sc are input to the inverter 13.
The second current determining block 16b of the d and q axis currents determining module 16 and the second block 18b of the switching signal generation module 18 may be implemented using dq-abc transformations. An integrator block 19 is configured to integrate the reference speed ω*=2πf*, thereby forming a reference phase θ* by continuously integrating ω*, which is input to the second current determining block 16b of the d and q axis currents determining module 16 and the second block 18b of the switching signal generation module 18.
The model based MTPA control approach disclosed above works well if the induction motor magnetization flux is assumed linear function of magnetizing current (constant magnetizing inductance). In reality, this is not true due to the saturation characteristics of motor magnetic flux. At partial speeds and/or low flux levels, the performance is optimum since the motor flux is not saturated at this operation condition. But at nominal speed or when the motor voltage is near the nominal value, due to magnetizing inductance saturation effects, the MTPA control approach as described above does not lead to optimum current operation.